Light-guide lights suitable for use in illuminated displays

ABSTRACT

A light-guide light ( 1 ) suitable for use in illuminated displays and signs comprises a housing ( 3 ) defining an optical cavity having first and second generally parallel major faces ( 5, 6 ), and a light source ( 11 ) positioned to direct light into the optical cavity from one side. The first major face ( 5 ) comprises a material (for example, a prismatic film) having coefficients of reflection and transmission that vary with the angle at which light is incident on the material. The second major face ( 6 ) comprises a narrow-scattering reflective material having a reflectance of at least 85%, for example a highly-efficient reflective material provided with a suitable textured pattern.

FIELD OF THE INVENTION

The present invention relates to a light-guide light capable ofproviding an illuminated panel and suitable for use, for example, inilluminated displays and signs and in other lighting applications.

BACKGROUND OF THE INVENTION

It is already known to use light guides to illuminate panels for generallighting purposes and also for display applications (e.g. forilluminating signs and advertisements, and also for illuminating liquidcrystal displays). In one form, often referred to as a light box, thelight guide comprises a hollow box-shaped structure defining an opticalcavity, and in another form it comprises a solid light-guiding plate. Inboth forms, a major surface of the guide can be illuminated by lightdirected into the guide in a direction generally parallel to that majorsurface, for example from at least one elongated light source or asimilar arrangement located adjacent an edge of the light guide(so-called “edge-lit light guides”).

Illuminated panels based on edge-lit light guides are generally thinnerthan those that are lit from behind and, as a result, are visuallyattractive and also particularly useful when the depth of the spaceavailable for a panel is restricted. They also offer the advantage thatthe light source is separated to some extent from the panel so that theheat input into the latter from the light source is reduced. Hollowlight guides would appear to offer further advantages for applicationsthat require the weight of the light guide to be kept as low as possiblebut, despite that, solid light guides have typically been more widelyused because they are comparatively simple to produce and are theeasiest way of transporting light.

Light guides in the form of hollow light boxes are described, forexample, in EP-A-0 490 279; 0 377 309; and 0 293 182; and in GB-A-2 310525. In each of those light boxes, a prismatic optical film is employedwith a view to achieving a more even distribution of light over thesurface that is being illuminated. Practical designs for light boxes,intended for use in illuminating graphic displays, are described in anApplication Bulletin entitled “Thin Light Box” and issued in March 1990by Minnesota Mining and Manufacturing Company of St. Paul, Minn., USAU.S. Pat. No. 6,080,467 describes an illuminated sign comprising a lightbox, the interior surfaces of which comprise a multi-layer reflectiveoptical film.

An illuminated sign, suitable for use on an automotive vehicle, isdescribed in WO 00/65277. The sign comprises a housing havingdiffusely-reflecting interior surfaces and a front sign face throughwhich light from inside the housing is transmitted, the light beingsupplied by a light fibre located on an interior surface of the housing.

International patent application WO01/71248 describes a hollow lightguide suitable for use in illuminating a graphic display. The front faceof the light guide comprises Scotch™ Optical Lighting Film and forms awindow through which light can leave the light guide. The rear face ofthe light guide comprises a highly-efficient specularly-reflectingoptical film printed with an array of dots in a diffusely-reflectingink. These dots form light-extraction elements and cause light to beemitted through the front face of the light guide. The arrangement ofthe dots on the rear face of the light guide is related to the size andshape of the light guide to yield a uniform illumination of the frontface.

There is a continuing demand for improved illuminated panels especially,but not exclusively, for display purposes. One problem with many displaypanels is that the panel is more brightly illuminated in the areaclosest to the light source, which detracts from the overall visualappearance and effectiveness of the illumination. Accordingly, there isa demand for improved uniformity in the illumination and for theelimination, from the illuminated panel, of any visible signs of thelocation and nature of the light source(s). It is also highly desirable,from an environmental and a cost point of view, that the amount of powerused for illumination purposes should be kept as low as possible.

SUMMARY OF THE INVENTION

The present invention is directed to the problem of providing alight-guide light which is suitable for display purposes and capable ofmeeting the demands for uniform illumination and efficiency, and whichcan be assembled comparatively easily in a variety of sizes.

The present invention provides a light guide comprising a housingdefining a light-guiding optical cavity having first and secondgenerally parallel major faces, and at least one light source arrangedto direct visible light into the cavity from one side, to be guidedbetween the first and second major faces, wherein:

-   -   (a) the first major face comprises a material having        coefficients of reflection and transmission that vary with the        angle at which light is incident on the material; and    -   b) the second major face comprises a narrow-scattering        reflective material having a total reflectance of at least 85%        for visible light incident on the surface at any angle;    -   whereby light from within the cavity is emitted substantially        uniformly across the first major surface.

The term “narrow-scattering reflective material” means a material thatreflects an incident collimated light beam into a broadened beam havinga dispersion angle of less than about 15°. The term “dispersion angle”means the angle between the direction of maximum intensity (I_(max)) ofreflected light and the direction of intensity with a value I_(max)/2,assuming an intensity of reflected light distribution curve that issymmetrical about the direction of I_(max). If the intensitydistribution curve of the reflected light is not symmetrical about thedirection of I_(max), the term dispersion angle as used herein means themean angle between the direction of I_(max) and a direction of intensityI_(max)/2. The broadened reflected beam may, or may not, exhibit apronounced peak in the direction of maximum intensity.

Light-guide lights in accordance with the invention can be producedcomparatively easily in different sizes in a manner that is appropriateto bespoke production, and can offer effective, uniform, and efficientillumination for display purposes and for other lighting applications.

BRIEF DESCRIPTION OF THE INVENTION

By way of example, embodiments of the invention will be described withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a light guide panel in accordance withthe invention;

FIG. 2 is a diagrammatic perspective view of a light guide, similar tothat shown in FIG. 1, the light guide being shown partly exploded;

FIG. 3 is a diagrammatic cross-sectional view, on the line III-III ofFIG. 2, of the light guide in assembled form;

FIG. 4 is a cross-sectional view, similar to FIG. 3, of another lightguide;

FIG. 5 illustrates a modification of the light guide of FIGS. 2 and 3;and

FIG. 6 is a diagrammatic transverse cross section of a lighting tubehousing for use with a light-guide in accordance with the invention.

DETAILED DESCRIPTION

The light-guide light 1 shown in FIG. 1 comprises a box-like housing 3defining an optical cavity. The housing 3 has opposed major faces 5, 6,and opposed narrow sides 7, 8 and 9, 10. An elongate light source 11 isarranged adjacent one of the narrow sides 7 to direct light into theoptical cavity in a direction generally parallel to the planes of themajor faces 5, 6. One of the major faces (the face 5) forms a windowthrough which light can be emitted from within the optical cavity andused for illumination purposes.

The optical cavity 13 inside the housing 3 is visible in thediagrammatic illustration of FIG. 3. The narrow side 7 of the housingadjacent the light source 11 comprises an optical sheet material 15forming a window through which light from the source 11 can enter thelight guide 1. Preferably, the sheet material 15 has a structuredsurface on the side remote from the light source, to redirect the lightfrom the source 11 and ensure that the light that passes through thiswindow enters the optical cavity 13 preferentially in a directiongenerally parallel to the planes of the faces 5, 6. The optical sheetmaterial 15 may, for example, have a structured surface comprising aseries of ridges and grooves formed by a plurality of paralleltriangular prisms. A similar use of sheet material of that type isdescribed in EP-A-0 293 182. In the light guide 1, the material 15 ispreferably oriented so that the prisms extend parallel to the elongatelight source. Suitable sheet material is available, under the tradedesignation “Scotch™ Optical Lighting Film”, from Minnesota Mining andManufacturing Company of St. Paul, Minn., USA.

The narrow side 8 of the light guide 1 opposite the window 15 has areflecting surface 17 on the side facing into the optical cavity 13.This reflecting surface, which is preferably a highly-efficientspecularly-reflecting surface, can be provided by any suitable materialbut is preferably provided by a multi-layer optical film of the typedescribed in U.S. Pat. No. 5,882,774 and WO97/01774. A suitable materialis the film available, under the trade designation “VM2000 RadiantMirror Film”, from Minnesota Mining and Manufacturing Company of St.Paul, Minn., USA.

The other two opposed narrow sides 9, 10 of the light guide also havereflecting surfaces 18 facing into the cavity (see FIG. 2). Thesereflecting surfaces 18 are preferably provided by a film materialavailable, under the trade designation “Light Enhancement Film”, fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA,although any other suitable reflecting material can be used. Generally,it has been found that a diffusely-reflecting material is preferablewhen the length/width ratio of these narrow sides is less than 10 andthat a specularly-reflecting material is preferable when this ratio isgreater than 10. It will be appreciated that this ratio corresponds tothe length/thickness ratio of the light guide 1 (otherwise known as its“aspect ratio”).

The front and rear faces 5, 6 of the light guide comprise materialsthat, preferentially, guide the light from the source 1.1 along theoptical cavity 13, between the faces and towards the edge 8, althoughthe front face 5 will also permit light to leave the optical cavity whenit is incident on the face 5 at certain angles, as described below.

More specifically, the front face 5, or window, of the light guidecomprises an optical sheet material 19 having coefficients of reflectionand transmission that vary with the angle at which light is incident onthe material. The material 19 has a smooth surface facing into theoptical cavity and, on the side facing away from the optical cavity, astructured surface comprising a series of ridges and grooves formed by aplurality of parallel triangular prisms whereby light incident on thematerial 19 while traveling along the optical cavity 13 will be totallyinternally reflected provided it is incident on the material 19 within apredetermined angular range. As such, the material 19 may be the same asthe material 15 and, in this case, the material is oriented so that theprisms extend in a direction at right angles to the direction of extentof the light source 11 as indicated in FIG. 2. A similar use of materialof that type is described in EP-A-0 293 182. To protect the prismaticstructures on the sheet material 19, a further panel 21 may bepositioned adjacent the material 19 on the outside of the light guidehousing. This further panel is not essential but, when provided, it maycomprise a sheet of clear material or opalescent light-diffusingmaterial. Use of an opalescent material may enhance even further theuniformity of the light that passes through the sheet material 19.

The rear face 6 of the light guide 1 comprises a sheet material 23 whichprovides a highly reflective-surface 24 facing into the optical cavity13, the reflective surface 24 being capable of causing limitedcontrolled spreading of an incident light beam into a broadenedreflected beam. Materials of this type are known under the generaldescriptions “scattering reflective materials” and can be furtherclassified as either “wide” or “narrow” scattering reflective materials,depending on the angular spread of the reflected beam (see “Daylightingin Architecture—A European Reference Book”, published by James andJames, London, 1993. ISBN 1-873936-214, at pages 4.3 to 4.5). In thelight-guide 1, the reflective surface 24 is a narrow scatteringreflector (meaning that it has a dispersion angle of less than about 15°or, more typically for the present application, between about 5° and15°) but should be such that its reflectivity is not reducedsubstantially for light that is incident in directions other than normalto the surface, and is at least 85% (preferably at least 90% and, mostdesirably, at least 98%). To achieve that, the reflective surface 24 maybe a highly-efficient reflective surface provided with a texturedpattern that is designed to spread the reflected light in the desiredmanner without substantially degrading the total reflectivity of thesurface. One example of a suitable scattering reflective material is thefilm material embossed with a sand-blast pattern that is available,under the trade designation “Radiant Light Film Embossed VM2000”, fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA Analternative sheet material is a highly reflective sheet metal material,for example sheet aluminium, formed with a suitable pattern to producethe desired spreading of the reflected light. In that case, a suitablepattern may be a pattern of dimples or bumps such as those produced bypeening the sheet metal.

In FIGS. 2 and 3, the light source 11 is shown as being located in athree-sided housing 25, the open side of which is positioned adjacentthe sheet material 15 forming the entry window of the light guide 1. Theuse of the sheet material 15 in the narrow side 7 of the light-guidehousing adjacent the light source 11, although preferred in thisarrangement, is not essential. The housing 25 is constructed to directas much light as possible from the light source 11 into the opticalcavity 13 and, to that end, the internal surfaces of the housing may becovered with a suitable highly-efficient, reflecting material, forexample a reflective paint or sheet material. Alternatively, the lightsource 1 could be provided with a parabolic reflector to direct thelight from the source towards the optical cavity 13, or it could bereplaced by a suitable apertured light source, or a combination of both.

The light guide 1 as described above functions as follows. Light fromthe source 11 (possibly following reflection or redirection at the wallsof the housing 25) enters the optical cavity 13 through the windowmaterial 15 and travels preferentially in a direction parallel to themajor surfaces 5, 6 of the light guide towards the surface 17 where itwill be reflected and returned. However, any light that is incident onthe rear surface 24 will be spread on reflection and some of that lightwill, as a consequence, subsequently impinge on the front face 5 of thelight guide in such a direction and at such an angle that it can passthrough the optical sheet material 19 and emerge from the light guide.In other words, the rear surface 24 performs a light scattering functionthat enables light to be emitted through the front face 5 of the lightguide while preserving the direction of light propagation within theoptical cavity. It has been found that the overall effect of theconstruction of the light guide 1 is to provide high level, uniform,illumination of the front face 5. The uniformity is particularly goodwhen the light guide 1 has an aspect ratio no greater than 10 but isalso acceptable at higher aspects ratios. When used to illuminate agraphic display, the latter is placed on the outside of the sheetmaterial 19 (i.e. adjacent the prisms) or on the outside of the panel 21(when present). If the panel 21 is a sheet of clear material, thegraphic display may be located between it and the sheet 19.

FIG. 4 illustrates a light guide 31 that is generally similar to theguide illustrated in FIGS. 2 and 3 but incorporates an additional lightsource 11′ positioned opposite to the light source 11 (i.e. adjacent thenarrow side 8 of the housing 3). To enable light from the source 11′ toenter the optical cavity 13, the side 8 of the housing 3 comprises anoptical sheet material 15′ forming a window, rather than the reflectingmaterial 17 of FIG. 3.

The light source 11′ is located in a three-sided housing 25′ similar tothat of the light source 11 but, like the light source 11, it couldalternatively be provided with a parabolic reflector to direct lightfrom the-source into the-optical cavity, or be replaced by a suitableapertured-light source, or a combination of both. The material 15′forming the window from the housing 25′ into the optical cavity 13 ispreferably the same as the optical sheet material 15.

The light guide 31 functions in a similar manner to the guide 1described above except that, in this case, light from both sources 11,11′ (possibly following reflection or redirection at the walls of theassociated housing 25, 25′) enters the optical cavity 13 through theassociated window material 15, 15′ and travels preferentially in adirection parallel to the major surfaces 5,6 of the light guide towardsthe light housing at the other end of the optical cavity where some ofthe light will be reflected and returned. Any light that is incident onthe rear surface 24 will be spread on reflection and some of that lightwill, as a consequence, subsequently impinge on the front face 5 of thelight guide in such a direction and at such an angle that it can passthrough the optical sheet material 19 and emerge from the light guide.As with the light guide 1 of FIGS. 2 and 3, it has been found that theoverall effect of the construction of the light guide 31 is to providehigh level, uniform, illumination of the front face 5, particularly whenthe light guide 31 has an aspect ratio no greater than 10 (although theuniformity is also acceptable at higher aspect ratios). It will be notedthat the rear surface 24 of the light guide 31 requires no modification,compared with the rear surface of the light guide 1, despite the factthat two light sources are used (which would not be the case, forexample, if the rear surface were provided with a printed array of lightextracting elements).

The use of a sheet material 23 for the rear face of the optical cavity13 of the light guides 1, 31 is advantageous because such a material iseasy to store and to handle prior to, and during, assembly of the lightguide. When in use in the light guide, the sheet material 23 preventslight from leaving the optical cavity 13 through the rear face 6 andthus enhances the illumination of the front face 5. In addition, anyscratches on the surface of the reflective sheet material (which mightarise, for example, during handling or assembly of the light guide) willnot adversely affect the uniform illumination of the front face 5. Onlyone form of sheet material 23 is required to produce light guides forilluminating panels in a comparatively wide range of differingsizes-within a particular range of aspect ratios (e.g. aspect ratioswithin the range of from 5 to 10). This, in turn, enables the assemblyof the light guides to be simplified and the assembly time to be reducedsince it is not necessary to design the face 24 of the sheet materialspecifically to suit the particular geometry of the light guide that isbeing produced.

A hollow light guide as described above with reference to FIGS. 1 to 3or 4 can be fabricated in such a way that it is comparativelylightweight. That is a particular advantage when the light guide islarge in size (for illuminating large signs, for example), andespecially when it is required to be installed in a less accessiblelocation. Of particular interest in the field of illuminated signs isthe fact that edge-lit light guides can be fabricated with depths assmall as 10 cm and even, depending on the size of the sign, as small as1 cm.

The light sources employed with the light guides 1, 31 are not requiredto have an elongate form as illustrated. Other light sources could beemployed including, for example, an array of light emitting diodes(LEDs).

The light guides illustrated in FIGS. 1 to 4 have been described aboveas being used to illuminate a graphic display but they could be used forother purposes including, for example, illuminating liquid crystaldisplays or signs or general illumination purposes.

Examples of illuminated signs incorporating light guides of the typeillustrated in FIGS. 1 to 3 will now be described.

EXAMPLE I

The housing 3 of the light guide 1, excluding the front major face 5,may be a one-piece vacuum-formed construction of any suitable material,for example PVC (polyvinylchloride). Alternatively, the housing may beformed from several pieces of, for example, an acrylic material, eachproviding one side of the housing, which are secured together in anysuitable manner. The housing is approximately 60×60×4.5 cm.

The internal surface of the rear major face 6 of the housing is coveredwith a sheet 23 of 3M™ “Radiant Light Film Embossed VM2000”. Theinternal surface of one narrow side 7 of the housing 3 is covered with asheet 15 of the above-mentioned “Scotch™ Optical Lighting Film”,arranged with the prisms facing into the housing and extending parallelto the long edges of this side of the housing. The internal surface ofthe opposite narrow side 8 of the housing 3 is covered with a sheet ofthe above-mentioned “VM2000 Radiant Mirror Film.” The internal surfacesof the remaining two narrow sides 9, 10 of the housing 3 are coveredwith the above-mentioned “Light Enhancement Film.” Alternatively, all ofthe internal surfaces 6, 8, 9 and 10 may be formed from theabove-mentioned “Radiant Light Film Embossed VM2000” material making itpossible to vacuum-form these elements of the housing 3 from that filmmaterial.

The housing 3 is closed with a sheet 19 of the above-mentioned “Scotch™Optical Lighting Film”, forming the front major face 5. The film isarranged so that the prisms are on the outside of the housing and extendbetween the narrow sides 7 and 8.

The light guide module thus formed was put into a sign housing andprovided with a 60 cm long, 14W fluorescent lighting tube located,within a high-reflectance housing 25, adjacent the narrow side 7 of thelight guide housing 3 and arranged to direct light into the latter. Itwas found that the front major face 5 of the housing 3 was illuminatedwith a high degree of uniformity and to a level sufficient to provideeffective illumination of a graphic image located in front of the face5.

EXAMPLE II

A light guide module similar to that described in Example I wasconstructed except that the housing 3 of the light guide was larger,having dimensions of approximately 120×180×6 cms. In addition, theoptical sheet material 15 on the narrow side 7 of the housing 3 wasomitted and the housing 25 for the lighting tube 11 was by a housing 40illustrated diagrammatically in FIG. 6 which also illustrates thedisposition of the housing relative to the narrow side 7 of the lightguide housing 3. The lighting tube housing 40, which is separate fromthe light guide housing 3, extends along the length of the lighting tube(indicated in FIG. 6 by the reference 41) and thus along length of theside 7 of the light guide housing. The housing 40 includes a backportion 42 that is located to the rear of the lighting tube 41, anddiverging flat sides 43 that extend from each front edge 44 of the backportion 42 towards the light guide housing 3. The diverging sides 43define an exit opening through which light from the lighting tube 41 canleave the housing 40. The back portion 42 of the housing is shaped sothat it partially surrounds, but is spaced from, the lighting tube 41and permits the latter to extend slightly forwards of the front edges 44as shown in the diagram. In FIG. 6, the back portion 42 is shown ascurved but it could, instead, comprise a series of planar sectionsapproximating to a curve. The inside surfaces of the housing 40 (i.e.both the back portion 42 and the sides 43) are covered with ahighly-efficient specularly-reflective material, for example theabove-mentioned “VM2000 Radiant Mirror Film.”

The lighting tube 41 is a small-diameter fluorescent tube, for example aT5 tube having a diameter of about 16 mm, and there is a gap of about 3mm between it and the back portion 42 of the housing 40. The sides 43 ofthe housing diverge at an angle of about 15° relative to a planeparallel to the major faces 5, 6 of the light guide housing 3 and extendforwardly of the lighting tube 41 until they meet the respective edgesof the major faces 5, 6 of the light guide housing 3 (i.e. the exitopening of the lighting tube housing 40 corresponds to, and isimmediately adjacent, the narrow side 7 of the light guide housing 3 tosupply light directly into the latter).

In an illuminated sign constructed in accordance with this example, itwas found that the front major face 5 of the housing 3 was illuminatedwith a high degree of uniformity and to a level sufficient to provideeffective illumination of a graphic image located in front of the face5.

Although the above examples, and the earlier description with referenceto the drawings, relate to the construction of light guide modules, itwill be appreciated that the same light guide construction could bebuilt directly into the housing of a sign as a permanent part of thelatter.

The use of a prismatic film material (such as the above-mentioned“Scotch™ Optical Lighting Film”) to form the front face 5 of the lightguide is also not essential although it is preferred. Any sheet materialhaving coefficients of reflection and transmission that vary with theangle at which light is incident on the material can be used to form thefront face 5 including, for example, a plane sheet of a transparentplastic material such as an acrylic material.

It will also be appreciated that other materials could be used for therear surface of the optical cavity, provided that they arenarrow-scattering reflective materials with a sufficiently highreflectance. For light guides having an aspect ratio of 10 or less, anarrow scattering reflective material that provides as broad a reflectedbeam as possible (i.e. a beam for which the dispersion angle is close to15°) will be preferred. However, as the aspect ratio increases,scattering reflective materials that produce narrower reflected beamswill provide an acceptable result. In some cases, it may be advantageousto use a material that spreads the reflected beam in a different manner(e.g. to produce a beam having pronounced asymmetry, being spread to amuch greater extent in a plane parallel to the front and rear surfaces5, 6 than in a plane parallel to the end faces 7, 8).

It was indicated above that light guides constructed as described withreference to FIGS. 1 to 3 exhibit a somewhat less uniform (althoughstill acceptable) light output when they have an aspect ratio of 10 ormore. In particular, when viewing the front face 5 of the light guide 1,a region of increased light intensity may be apparent adjacent to thelight source 11. This “edge glow” is generally more apparent if theprismatic film 19 is replaced by a plane sheet of transparent plasticmaterial as described above, but can be reduced in a comparativelysimple manner by applying light-absorbing elements to the inside face ofthe sheet material 19 (i.e. the face directed into the optical cavity13) adjacent the light source. The light absorbing elements may, forexample, be printed elements (e.g. dots) formed using a suitable ink(e.g. an opaque black ink with a gloss reflection). The surface coverageof the light absorbing elements is highest at the edge of the sheet 19immediately adjacent the light source 11 (e.g. 70% coverage of thesurface area) and decreases linearly to zero at a distance of about 150mm from that edge. This is illustrated in FIG. 5, which shows a region30 of light absorbing elements on the rear face of the sheet material 19adjacent the light source 11. The light absorbing elements can beapplied directly to the internal surface of the face 5 of the lightguide 1 or they can be applied to a separate sheet of transparentmaterial (e.g. vinyl) that is then laminated to the internal surface ofthe face 5: in each case, it has been found that the light absorbingelements are not discernible when an illuminated sign in which the lightguide is incorporated is being viewed.

It has been found that the arrangement of light absorbing elementsdescribed above is effective for most sign dimensions and can,accordingly, be provided as a standard part of all light guides ifrequired. If a light guide of the type shown in FIG. 4 is used, then asimilar arrangement of light absorbing elements will also be requiredadjacent the second light source 11′.

1-12. (canceled)
 13. A light-guide light comprising a housing defining a light-guiding optical cavity having first and second generally parallel major faces, and at least one light source arranged to direct visible light into the cavity from one side, to be guided between the first and second major faces, wherein: (a) the first major face comprises a material having coefficients of reflection and transmission that vary with the angle at which light is incident on the material; and (b) the second major face comprises a narrow-scattering reflective material having a total reflectance of at least 85% for visible light incident on the surface at any angle; whereby light from within the cavity is emitted substantially uniformly across the first major surface.
 14. A light-guide light as defined in claim 1, wherein the second major surface has a reflectance of at least 98% for visible light incident on the surface at any angle.
 15. A light-guide light as defined in claim 1, wherein the second major surface comprises a specularly-reflecting material having a light-spreading structure formed thereon.
 16. A light-guide light as defined in claim 1, wherein the second major surface comprises an optical film or sheet metal.
 17. A light-guide light as defined in claim 1, wherein the second major surface comprises a sheet material laminated to an internal surface of the housing.
 18. A light-guide light as defined in claim 1, further comprising a second light source arranged to direct light into the cavity from the end opposite the first-mentioned light source, to be guided between the major faces.
 19. A light-guide light as defined in claim 1, wherein the first major face comprises one side of a sheet material that has a structured surface comprising a plurality of parallel prisms on the side remote from the optical cavity.
 20. A light-guide light as defined in claim 1, wherein the first major face comprises one side of a planar sheet of transparent material.
 21. A light-guide light as defined in claim 1, further comprising light-absorbing elements applied to the first major face in a region adjacent the/each light source.
 22. A light-guide light as defined in claim 1, wherein a display that is to be illuminated is positioned outside the optical cavity in the path of light emitted through the first major face.
 23. A light-guide light as defined in claim 1, wherein the light source is an elongate source that extends along the length of the side of the cavity through which it directs light into the cavity; the light source having an elongate housing comprising: a back portion that is located to the rear of the lighting source, and is shaped so that it partially surrounds, but is spaced from, the lighting source, and diverging sides that extend from each front edge of the back portion towards the said side of the light guide cavity and define an exit aperture for light from the elongate housing; wherein the internal surfaces of the back portion and the sides of the light source housing comprise a reflective material.
 24. A light-guide housing as defined in claim 11, wherein the diverging sides are so arranged that the exit aperture corresponds to, and is immediately adjacent, the said side of the optical cavity to direct light into the latter. 